add tests for ubuntu 21

Signed-off-by: Adam Warner <me@adamwarner.co.uk>

test/_ubuntu_21.Dockerfile

@@ -0,0 +1,18 @@
+FROM buildpack-deps:hirsute-scm
+
+ENV GITDIR /etc/.pihole
+ENV SCRIPTDIR /opt/pihole
+
+RUN mkdir -p $GITDIR $SCRIPTDIR /etc/pihole
+ADD . $GITDIR
+RUN cp $GITDIR/advanced/Scripts/*.sh $GITDIR/gravity.sh $GITDIR/pihole $GITDIR/automated\ install/*.sh $SCRIPTDIR/
+ENV PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:$SCRIPTDIR
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN true && \
+    chmod +x $SCRIPTDIR/*
+
+ENV PH_TEST true
+ENV OS_CHECK_DOMAIN_NAME dev-supportedos.pi-hole.net
+
+#sed '/# Start the installer/Q' /opt/pihole/basic-install.sh > /opt/pihole/stub_basic-install.sh && \

test/tox.ubuntu_21.ini

@@ -0,0 +1,8 @@
+[tox]
+envlist = py37
+
+[testenv]
+whitelist_externals = docker
+deps = -rrequirements.txt
+commands = docker build -f _ubuntu_21.Dockerfile -t pytest_pihole:test_container ../
+           pytest {posargs:-vv -n auto} ./test_automated_install.py